The present invention relates to a method and an apparatus for expanding a band of an audio signal, capable of reproducing an audio signal pleasant to the human ear by improving the quality of a reproduced sound of an audio signal reproduced by audio equipment, and in particular, the quality of a reproduced sound of high audio frequencies. More particularly, the present invention relates to a method and an apparatus for expanding a band of an input audio signal by performing digital processing for the input audio signal.
The Japanese patent laid-open publication No. 9-36685 discloses an audio signal reproducing apparatus of the prior art for combining an analog audio reproduced signal with a signal having a frequency spectrum exceeding the highest audio frequency of a reproduction frequency band or the highest limit of the high audio frequency of an audible frequency band. A configuration of the audio signal reproducing apparatus is shown in FIG. 17. Referring to FIG. 17, the audio signal reproducing apparatus is constituted by comprising a buffer amplifier 91, a filter circuit 92, an amplifier 93, a detector circuit 94, a time constant circuit 95, a noise generator 96, a filter circuit 97, a multiplier 98 and an adder 99.
First of all, an audio signal is inputted to the buffer amplifier 91 from an input terminal T1, and then, is divided into two audio signals. One divided audio signal is inputted directly to the adder 99, whereas another divided audio signal is inputted to the filter circuit 92 which is of a high-pass filter or a band-pass filter. The filter circuit 92 band-pass-filters only a specific-band signal of the input audio signal, allows the signal to pass through the filter circuit 92, and then, outputs the same signal to the amplifier 93. The amplifier 93 amplifies the input audio signal to a predetermined appropriate level, and then, outputs the amplified signal to the detector circuit 94 having the time constant circuit 95. The detector circuit 94 detects an envelope level of the audio circuit by, for example, envelope detection of the input audio signal. Then, the detector circuit 94 outputs a level signal indicative of the detected envelope level to the multiplier 98 as a level control signal for controlling a level of a noise component to be added to the original audio signal.
On the other hand, a noise component generated by the noise generator 96 is inputted to the filter circuit 97 which is of a high-pass filter or a band-pass filter. The filter circuit 97 allows passage of a noise component of a frequency band of 20 kHz or more, and then, outputs the noise component to the multiplier 98. The multiplier 98 multiplies the input noise component by the level control signal from the detector circuit 94, generates a noise component having a level proportional to the level indicated by the level control signal, and then, outputs the generated noise component to the adder 99.
Furthermore, the adder 99 adds the noise component from the multiplier 98 to the original audio signal from the buffer amplifier 91, generates the audio signal having the added noise component, and then, outputs the audio signal through an output terminal T2. In this case, a time constant of the time constant circuit 95 is selected so as to have a predetermined value, and this leads to adapting the noise component generated by the noise generator 96 to characteristics of the human sense of hearing and thus enhancing an effect of improving sound quality of the audio signal.
As described above, a high-frequency range is expanded by adding random noise proportional to an output level of high audio frequencies of the original audio signal to the original audio signal. However, the above-mentioned audio signal reproducing apparatus of the prior art has the following problems.
(1) The sound is unpleasant to the ear on the quality of sound since a spectral structure of a high-frequency signal of an additional noise component is different from that of a musical sound signal.
(2) Since the audio signal reproducing apparatus of the prior art comprises an analog circuit, the apparatus has the following problems. That is, the performance of the apparatus changes due to variations in parts of the analog circuit and temperature properties. Consequently, deterioration in sound quality occurs each time when an audio signal passes through the analog circuit. Moreover, improvement in precision of the filter circuit constituting the analog circuit causes an increase in the scale of the filter circuit and thus an increase in the manufacturing cost.
(3) Further, when a signal having a single spectrum such as a sinusoidal wave is inputted to the apparatus, a random noise component is added to the signal. Therefore, the measurement of signal characteristics results in marked deterioration in the signal characteristics.
It is an essential object of the present invention to solve the above-mentioned problems, and provide a method and an apparatus for expanding a band of an audio signal, which eliminate the unpleasantness of a sound, cause no deterioration in sound quality, cause little variation in performance of the apparatus, and reduce manufacturing cost as compared to the prior art.
It is another object of the present invention to solve the above-mentioned problems, and provide a method and an apparatus for expanding a band of an audio signal, in which the measurement of signal characteristics does not result in deterioration in a signal even if a sinusoidal signal is inputted to the apparatus.
According to the present invention, there is provided a method for expanding a band of an audio signal including the steps of:
oversampling a digital audio signal of a first band having a predetermined maximum frequency with a sampling frequency that is two or more times the maximum frequency, and low-pass-filtering an oversampled digital audio signal so as to eliminate aliasing noise caused by the oversampling, and outputting a low-pass-filtered digital audio signal;
calculating a spectrum intensity of a predetermined band of the low-pass-filtered digital audio signal, and outputting a signal indicating the calculated spectrum intensity;
generating an expanded signal having frequency components of a second band higher than the first band;
controlling a level of the expanded signal in response to the signal indicating the calculated spectrum intensity; and
adding the expanded signal having the controlled level to the low-pass-filtered digital audio signal, and outputting a digital audio signal of addition result.
In the above-mentioned method, the step of generating the expanded signal preferably includes the steps of:
distorting the digital audio signal by performing non-linear processing on the low-pass-filtered digital audio signal with a non-linear input and output characteristic, and generating a digital signal having higher harmonic components of the digital audio signal; and
high-pass-filtering at least frequency components equal to or higher than the second band, from the digital signal having the higher harmonic components, and outputting a high-pass-filtered signal as an expanded signal.
In the above-mentioned method, the step of generating the expanded signal preferably includes the steps of:
generating a dither signal having a predetermined probability distribution for an amplitude level; and
high-pass-filtering at least frequency components equal to or higher than the second band, from the dither signal, and outputting a high-pass-filtered signal as an expanded signal.
In the above-mentioned method, the step of generating the expanded signal preferably includes the steps of:
distorting the digital audio signal by performing non-linear processing on the low-pass-filtered digital audio signal with a non-linear input and output characteristic, and generating a digital signal having higher harmonic components of the digital audio signal;
high-pass-filtering at least frequency components equal to or higher than the second band, from the digital signal having the higher harmonic components, and outputting a high-pass-filtered signal;
generating a dither signal having a predetermined probability distribution for an amplitude level;
high-pass-filtering at least frequency components equal to or higher than the second band from the dither signal, and outputting a high-pass-filtered signal; and
adding the two high-pass-filtered signals, and outputting a signal of addition result as an expanded signal.
The above-mentioned method preferably further includes the step of low-pass-filtering the expanded signal with a filter characteristic that is either one of a predetermined 1/f characteristic and a predetermined 1/f2 characteristic, prior to the step of controlling the level.
In the above-mentioned method, the step of generating the dither signal preferably includes:
a plurality of steps of generating a plurality of pseudo noise sequence noise signals independent of each other, respectively; and
a step of adding the plurality of pseudo noise sequence noise signals, generating a dither signal of addition result having a probability density of either one of a Gaussian distribution and a bell-shaped distribution for an amplitude level, and outputting the dither signal as an expanded signal.
The above-mentioned method preferably further includes the steps of:
calculating spectrum intensities of a plurality of predetermined bands of the low-pass-filtered digital audio signal, and judging whether or not the digital audio signal has a single spectrum in accordance with the calculated spectrum intensities of the plurality of bands; and
switching over so as to output the expanded signal when judging that the digital audio signal does not have any single spectrum, and switching over so as not to output the expanded signal when judging that the digital audio signal has a single spectrum.
According to the present invention, there is provided an apparatus for expanding a band of an audio signal comprising:
filtering means for oversampling a digital audio signal of a first band having a predetermined maximum frequency with a sampling frequency that is two or more times the maximum frequency, and low-pass-filtering the oversampled digital audio signal so as to eliminate aliasing noise caused by the oversampling, and outputting a low-pass-filtered digital audio signal;
first spectrum analyzing means for calculating a spectrum intensity of a predetermined band of the low-pass-filtered digital audio signal outputted from the filtering means, and outputting a signal indicating the calculated spectrum intensity;
expanded signal generating means for generating an expanded signal having frequency components of a second band higher than the first band;
level controlling means for controlling a level of the expanded signal in response to the signal indicating the calculated spectrum intensity outputted from the first spectrum analyzing means; and
first adding means for adding the expanded signal whose level is controlled by the level controlling means to the digital audio signal outputted from the filtering means, and outputting a digital audio signal of addition result.
In the above-mentioned apparatus, the expanded signal generating means preferably comprises:
non-linear processing means, having a non-linear input and output characteristic, for distorting the digital audio signal by performing non-linear processing on the digital audio signal outputted from the filtering means, and generating a digital signal having higher harmonic components of the digital audio signal; and
a first high-pass filter for high-pass-filtering at least frequency components equal to or higher than the second band, from the digital signal having the higher harmonic components outputted from the non-linear processing means, and outputting a high-pass-filtered signal as an expanded signal.
In the above-mentioned apparatus, the expanded signal generating means preferably comprises:
dither signal generating means for generating a dither signal having a predetermined probability distribution for an amplitude level; and
a second high-pass filter for high-pass-filtering at least frequency components equal to or higher than the second band, from the dither signal outputted from the dither signal generating means, and outputting a high-pass-filtered signal as an expanded signal.
In the above-mentioned apparatus, the expanded signal generating means preferably comprises:
non-linear processing means, having a non-linear input and output characteristic, for distorting the digital audio signal by performing non-linear processing on the digital audio signal outputted from the filtering means, and generating a digital signal having higher harmonic components of the digital audio signal;
a first high-pass filter for high-pass-filtering at least frequency components equal to or higher than the second band, from the digital signal having the higher harmonic components outputted from the non-linear processing means, and outputting a high-pass-filtered signal;
dither signal generating means for generating a dither signal having a predetermined probability distribution for an amplitude level;
a second high-pass-filter for high-pass-filtering at least frequency components equal to or higher than the second band, from the dither signal outputted from the dither signal generating means, and outputting a high-pass-filtered signal; and
second adding means for adding the signal outputted from the first high-pass filter to the signal outputted from the second high-pass filter, and outputting a signal of addition result as an expanded signal.
The above-mentioned apparatus preferably further comprises a low-pass filter, having a filter characteristic that is either one of a predetermined 1/f characteristic and a predetermined 1/f2 characteristic, for low-pass-filtering the expanded signal, and outputting a low-pass-filtered signal to the level controlling means.
In the above-mentioned apparatus, the dither signal generating means preferably comprises:
a plurality of noise signal generating circuits for generating a plurality of pseudo noise sequence noise signals independent of each other, respectively; and
third adding means for adding a plurality of pseudo noise sequence noise signals generated by the noise signal generating circuits, generating a dither signal of addition result having a probability density of either one of a Gaussian distribution and a bell-shaped distribution for an amplitude level, and outputting the dither signal as an expanded signal.
The above-mentioned apparatus preferably further comprises:
second spectrum analyzing means for calculating spectrum intensities of a plurality of predetermined bands of the digital audio signal outputted from the filtering means, and judging whether or not the digital audio signal has a single spectrum in accordance with the calculated spectrum intensities of the plurality of bands; and
switching means over for switching so as to output the expanded signal to the first adding means when the second spectrum analyzing means judges that the digital audio signal does not have any single spectrum, and switching over so as not to output the expanded signal to the first adding means when the second spectrum analyzing means judges that the digital audio signal has a single spectrum.
Therefore, according to the present invention, the apparatus for expanding the band of the audio signal is constituted by a digital signal processing circuit comprising the filtering means, the first adding means, the first spectrum analyzing means, the level controlling means and the expanded signal generating means. Therefore, the present invention can provide a method and an apparatus for expanding the band of the audio signal, which cause little variation in performance of the apparatus and reduce the manufacturing cost as compared to the prior art.
Moreover, the level of addition of an expanded signal is controlled in accordance with the high-frequency spectrum intensity of an input digital audio signal from the first spectrum analyzing means. Furthermore, the expanded signal passed through the low-pass filter having either one of a 1/f characteristic and 1/f2 characteristic is used. Therefore, the expanded signal having a natural sound close to a musical sound signal can be added to the input signal. Accordingly, there is no unpleasantness of a sound and no deterioration in sound quality.
Furthermore, the present invention comprises the second spectrum analyzing means and the switching means, and therefore, the present invention can provide a method and an apparatus for expanding a band of an audio signal, in which the measurement of signal characteristics does not result in deterioration in a signal even if a sinusoidal signal is inputted to the apparatus.